在△ABC中,a=3,b=7,c=5,则cosB= .
考点分析:
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已知
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+
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+
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281011_ST/2.png)
=
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,|
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|=2,|
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|=3,|
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|=
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,则向量
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281011_ST/8.png)
与
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之间的夹角
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281011_ST/10.png)
为( )
A.30°
B.45°
C.60°
D.以上都不对
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已知
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281010_ST/0.png)
=(2,-1,3),
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281010_ST/1.png)
=(-1,4,-2),
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281010_ST/2.png)
=(7,5,λ),若
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281010_ST/3.png)
、
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281010_ST/4.png)
、
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281010_ST/5.png)
三向量共面,则实数λ等于( )
A.
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281010_ST/6.png)
B.
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281010_ST/7.png)
C.
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D.
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与 向量
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281009_ST/0.png)
共线且满足方程
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281009_ST/1.png)
的向量
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281009_ST/2.png)
为( )
A.不存在
B.-2
C.(-4,2,-4)
D.(4,-2,4)
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若椭圆的两焦点为(-2,0)和(2,0),且椭圆过点
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,则椭圆方程是( )
A.
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281008_ST/1.png)
B.
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C.
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D.
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已知抛物线y
2=2px(p>0)的经过焦点的弦AB的两端点坐标分别为A(x
1,y
1)、B(x
2,y
2),则
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的值一定等于( )
A.4
B.-4
C.p
2D.-p
2
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